The present invention relates to a method and apparatus for controlling a pump of a braking system for delivering a pressure medium.
German Published Patent Application No. 195 48 248 discusses a method and an apparatus for controlling a pump of an electrohydraulic braking system. In this method and apparatus, the hydraulic fluid is injected from a pressure accumulator through valves into the wheel brake cylinder, the pressure accumulator being charged by a pump. In order to make charging of the pressure accumulator as noiseless as possible, the pump is activated with a demand-based interpulse ratio of a PWM signal, depending on the required volume of hydraulic fluid and a pressure in the pressure accumulator. The PWM signal is modified, as a function of a definable pressure value, with the deviation of the actual pressure from that value (pressure accumulator hysteresis). In this context, a plurality of fixed pressure threshold values are used for activation at appropriate delivery capacity. German Published Patent Application No. 195 48 248 specifically describes the activation of a pump in an electrohydraulic braking system. A braking system comparable thereto is also described in SAE Paper 960991.
Also known are methods and apparati in which the return flow pumps in hydraulic braking systems are activated as a function of the generator voltage. Hydraulic braking systems of this kind are known, for example, from the offprint of ATZ 95 (1993) Vol. 11, or from German Published Patent Application No. 195 46 682. German Published Patent Application No. 42 32 130 describes, for this purpose, a method and an apparatus for controlling an electric motor-driven hydraulic pump that generates auxiliary pressure for a braking system having an antilock braking control system and/or a drive slip control system. For that purpose, it is equipped with a variable activation clock cycle made up of a pulse/interpulse sequence. The voltage induced in generator fashion by the pump motor during the interpulses is analyzed as an indication of the pump rotation speed. By differentiating this generator voltage (as the rotation speed actual value) with a reference variable for the pump motor rotation speed created in an antilock braking or drive slip control system, a difference variable is then made available to a downstream controller. The output signal of the controller is used to create the pulse-width modulated adjusting signal for pump activation. The drive motor of the hydraulic pump is switched on and off in time with this pulse-width modulated adjusting signal.
It has been found that the known method and the corresponding apparati are not capable of yielding optimum results in every respect. With the known braking systems, a volume demand request is always necessary in order to activate the pump. A volume demand request occurs, for example, during the braking operation when volume is withdrawn from the braking circuit to be injected into the wheel brake cylinder. But according to the existing art, it is only after the pressure has fallen below a threshold value that the volume demand request resulting therefrom is recognized. The result is that the delivery capacity associated with the respective threshold value is utilized relatively late, and the pressure has then already fallen below the threshold value.
In hydraulic braking systems, the generator voltage is used, as a value representing rotation speed, to activate the pump. Because of the dead-time behavior in the conversion of the generator voltage and pressure conditions, however, here again it is impossible to prevent the pressure from falling below its limits. A further disadvantage with both aforesaid methods is the fact that in the case of a very slow pressure drop, e.g. at low temperatures or when the ambient temperature is falling, the reaction to the pressure change occurs very late, in fact not until the pressure has fallen below the threshold value.
Creation of the activation signal, in particular of the switch-on signal of the pump, is dependent on at least one condition. One condition for creating an activation signal for activating the pump of the braking system is constituted, according to the present invention, at least as a function of a pressure gradient variable. This pressure gradient variable represents an actual value of a pressure gradient of the pressure medium in the braking system. The condition for creation of the activation signal for activating the pump is referred to as the xe2x80x9cdependency modexe2x80x9d and is configured, for example, as a comparison of the pressure gradient variable to various pressure gradient threshold values, and/or as an analysis of a characteristics diagram with the pressure gradient variable as one characteristics diagram variable. Selection of the various pressure gradient threshold values is accomplished with the aid of at least one reference wheel brake pressure threshold, which also indicates whether or not a volume demand in terms of the pressure medium exists in the braking system.
With the approach according to the present invention, the use of the pressure gradient variable as the basis for the activation or switch-on condition of the pump of the braking system prevents the pressure from falling below the lowest threshold value, or makes possible much less of a decrease below it. By monitoring the pressure gradient it is moreover possible, even without a volume demand request in the braking system, to compensate for the influence of disturbances, for example a change in temperature and a resulting change in the pressure of the pressure medium, and thus to keep the pressure at a desired pressure value. With the use of the pressure gradient as the switch-on condition, it is moreover possible to achieve a lower average pressure and thus a lower average pump output with a longer-duration phase of high volume demand, since it is the information about the change in pressure that is already being used, and not exclusively the fact that pressure has fallen below the threshold value. The use of fixed pressure threshold values in addition to the pressure gradient threshold values results in a higher degree of reliability in terms of the necessary pressure conditions. A further advantage is that because the reference wheel brake pressure threshold values are stipulated as the selection criterion for the respective pressure gradient threshold value, the pressure gradient can be established in demand-oriented and situation-dependent fashion.